A concavo-convex lens has refractive index 1.5 and the radii of curvature of its surfaces are 10 cm and 20 cm. The concave surface is upwards and is filled with oil of refractive index 1.6. The focal length of the combination will be
a. 18.18 cm
b. 15 cm
c. 22 cm
d. 28.57 cm